Integrated data processing system



March 17, 1970 P. L. RANDLEV INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21. 1961 12 Sheets-Sheet 1 FIGJ m mm J. E A a o m W mm a T w r m m w J m CN A EOR IN.- R R E D m m Y M B COMPUTER BUFFER PROCESSING COMPUTER l5, mroamnou |s,a|r SAMPLE um:

ATTORNEYS March 17, 1970 P. L. RANDLEV 25,332

INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21, 1961 12 Sheets-Sheet 8 FIG. 2A

TERMINAL CONTROLS INFORMATION LINE 205 252 ZOI T DELAY masponse (Form slop. Iob lino spun) 244 Mechanism (Print or OUTPUT DECODER INPUT CONDITION CONTROL TO B FROM CONTROL COMPUTER OR BUFFER March 17, 1970 P. L. RANDLEV Re. 26,

IITEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21. 1961 12 Sheets-Sheet 5 TRANSACTION CODE END SERVICE RBJUEST ENCODER OUTPUT WDE CONTROL FIG.2B

March 17, 1970 Original Filed Nov. 21. 1961 P. L. RANDLEV INTEGRATED DATA PROCESSING SYSTEM 12 Sheets-Sheet 4 FIG.3

TERMINAL BUFFER SCAN CONT.

a ER 3|!) 320 DECODI F SSI) CHARACTER CHARACTER CHARACTER CHARACTER REGISTER REGISTER REGISTER REGISTER L; J p' 32|\ DECODER TERMINAL SIDE I WRITE READ/WRITE READ READ READ CIRCUITS CIRCUITS CIRCUITS CIRCUITS CIRCUITS 302,] soy] 304, I 305,] 30s, I

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March 17, 1970 P. L. RANDLEV Re. 26,832

INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21, 1961 12 Sheets-Sheet '7 FIG] D0 ANY OF THE BUFFERS REQUIRE SERVICE INTERRO GATE ALL FOUR BUFFERS PROVIDE A 2.5 ms noes THE NEXT TERmNAL MEMO- YES TERMINAL REQUIRE GATION CYCLE SERVICE STEP To THE NEXT TERMI- NAL AT THE END OF FIG IOpSEC MACHINE CYCLE L IIOIl IIOT) SIX GROUP CONTROL GROUP A CLOCK PU SES T a P GROUP A NOT ACTIVE PULSE n f IIO2 N08 1 r T & GROUP 8 CLOCK PULSES I GROUP B NOT ACTIVE PULSE H03 H09 T a GROUP 0 CLOCK PULSES GROUP C NOT ACTIVE PULSE T f a N w,

GROUP D NOT ACTIVE PULSE IIO5) IIII) a GROUP E CLOCK PULSES GROUP E NOT ACTIVE PULSE N06) IIIZ) T a GROUP F CLOCK PULSES I GROUP F NOT ACTIVE PULSE I IO pun SIX GROUP CLOCK PULSE March 17, 1970 P. RANDLEV INTEGRATED DATA PROCESSING SYSTEM Original. Filed Nov. 21. 1961 12 Sheets-Sheet a TAOI INFO. LINE TADZ moluz mro. umz TFOI 8 F|G.8 CHARACTER REGISTERS AND ASSOCIATED CIRCUITRY muff-Sou March 17, 1970 P. 1.. RANDLEV Re. 26,832

INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21, 1961 12 Sheets-Sheet 9 90k F um I i a EIGHT CLOCK ans BIT SAMPLE ml CHARACTER REGISTER OK I 3 TRANSFER mronumou TAOI a I CHARACTER READY I I 9|2 ITAOI REQUIBES I SEEE'EST OR I SERVICE mm. LINE um I m USE OR I a GRANT [SERVICE mm SERVICE I 907 T 905 (908 909/ am I REQUEST T TERM OUTPUT coumoi '0 R I I IOUTPUT comnoumssen I I T TPUT EON I 905 INPUT EOM CONTROL COMPUTER TERMINAL SAMPLING March 17, 1970 P. L. RANDLEV 26,832

INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21, 1961 12 Sheets-Sheet l0 F|G.|OA TWENTY TERMINAL GROUP A CONTROL ermjnul requlres servlce GROUP A cwcK PULSES & GROUP A NOT ACTIVE March 17, 1970 P. L. RANDLEV INTEGRATED DATA PROCESSING SYSTEM Original Filed Nov. 21. 1961 SERVICE REQUEST I FROM BUFFER #l SIGNALTRACKI OUTPUT CONTROL PROGRAM BUFFER #l OUTPUT EOM SERVICE REQUEST OUTPUT CONTROL PROGRAM BUFFER 2 SERVICE REQUEST I T I z 5 IZOZ IN USE GRANT SERVICE 12 Sheets-Sheet 12 BUFFER I TRIGGER GRANT SERVICE BUFFER #2 BJFFERS REQUIRE SERVICE OUTPUT CONTROL 0R T 203 PROGRAM BUFFER #3 T OUTPUT Eon T Jam INPUT EOM A 1 T GRANT SERVICE BUFFER-#4 SERVICE REQUEST I l OUTPUT CONTROL 0R T |2o4 PROGRAM BUFFER #4 I205 OUTPUT EOM INPUT EOM I 3 j was a B#2 FOUR POSITION I I224 CLOSED RING I a#3 1 T man was I220 IO gsec CLOCK PULSES a q FFERS m &

SERVICE I227 I aurrsas ACTIVE I230 I23I I J a IOpsec-SIX GROUP CLOCK PULSES I0 2580 CLOCK PULSES I BUFFER SAMPLE CONTROL Unitcd States Patent 26,832 INTEGRATED DATA PROCESSING SYSTEM Peter L. Randlev, 'Croton-on-Hudson, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of Delaware Original No. 3,245,045, dated Apr. 5, 1966, Ser. No.

153,880, Nov. 21, 1961, which is a continuation of application Ser. No. 719,791, Apr. 1, 1968. Application for reissue Mar. 20, 1.969, Ser. No. 831,787

Int. Cl. H04j 3/00 US. Cl. 340172.5 17 Claims Matter enclosed in heavy brackets II] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A data processing system which comprises a central computer and a plurality of remote terminals. Each of the remote terminals includes an input/output device which is adapted to transmit messages or to receive messages from the central computer, certain of the remote terminals being directly connected to the central computer. Each remote terminal has means associated therewith for generating a terminal service request signal when there is a message to be transmitted to or from a particular remote terminal. A plurality of buffers are included, others of the remote terminals being connected by groups to the central computer through the bufiers, a different group of the other terminals being connected through each of the bufiers. Means are provided for generating a bufier service request signal at any bufler when there is a message to be transmitted to or from a terminal connected to the bufler and means are also included for successively scanning each of the directly connected terminals to detect the presence of a terminal service request signal. There are further provided normally inoperative means or successively scanning the bufiers to detect the presence of a bufier service request signal and control means eflective after the scanning of each of the directly connected terminals to respond to the presence of a bufler service request signal for any one or more of the other remote terminals for interrupting the operation of the direct terminal scanning means and initiating an operation of the bufier scanning means. The aforementioned control means is responsive to the termination of the last buffer service request signal to restore the operation of the direct terminal scanning means and to stop the operation of the bufier scanning means. Means are further included, responsive to the detection by either of the scanning means of a service request signal for transferring information between the central computer and the remote terminal to which the detected service request signal pertains.

This invention relates to integrated data processing systems and more particularly to an integrated data processing system particularly suitable for use in providing complete data processing service to a manufacturing concern.

In such a system it is desirable that all of the records, such as, for example, payroll, production and inventory records, be stored and processed by a central computer. It is necessary that a number of remote terminals have access to the information stored in the computer and be able to insert new information into the computer and initiate transactions at the computer. In many cases these remote terminals are in close proximity to the control computer and can be directly connected to the central computer. However, many of the remote locations are widely separated geographically from the central computer and, in order to conserve communication lines, it is necessary to connect some of these remote terminals to a butter and connect only the butter to the central computer over a communication line.

When there are a great number of remote terminals there is a problem of providing service to all of the terminals in the shortest possible amount of time. The scanning and servicing of remote terminals is a particular problem when some of the terminals are connected directly to the computer and some of the terminals are connected through a butter to the computer. In the case of the directly connected terminals it is desirable that the terminals be able to transmit a message directly to the computer without transmitting a terminal identification character. This requires that the terminals be scanned in a predetermined sequence so that the central computer is always informed of the directly connected terminal that is being serviced. In the case of the buffers, it is desirable that any buffer requiring service be granted priority because each of the buffers is servicing a number of terminals and another terminal serviced by that butler may be held up if the buffer is not granted priority. For this reason, it is desirable to intersperse the scanning of the directly connected terminals with the scanning of the bullets. That is, after one directly connected terminal is scanned, it is desirable to scan all of the buffers to see if any require service and (if they do, then scan all the buffers. This interspersed scanning and the requirement that the control computer be able to tabulate exactly the directly con nected terminal presently being serviced presents a difficult problem in such a data processing system.

Another problem in such data processing systems is that a particular remote terminal should not be allowed to have access to all of the information in the central computer. That is, remote terminals in the production department should be able to change and have access to only production information and should not, for example, be permitted to change or have access to payroll information. Thus, it is desirable to establish a rightto-know in regard to each remote tcrminals access to certain information in the central computer.

These and other problems associated with such an integrated data processing system are solved by the present invention which is directed particularly to the interconnection of the central computer and the input-output devices at each of the remote terminals.

Accordingly, it is an object of the present invention to provide an improved integrated data processing system having a plurality of remote terminals connected directly to a central computer and a plurality of remote terminals connected through buffers to the central computer with provision to scan all of the remote terminals and service the terminals quickly and efliciently.

It is another object of the present invention to provide an improved integrated data processing system in which a plurality of remote terminals are scanned by a central computer at a high scanning rate until a plurality of the terminals requiring service have been scanned and are being serviced by the computer at which time the scan assumes a dillerent, slower rate.

It is a still further object of the present invention to provide an improved integrated data processing system having a plurality of remote terminals directly connected to the computer and a number of remote terminals connected through buffers to the computer and in which priority is given to request for service by one of the butters so that the butler interrogations are interspersed with interrogations of directly connected terminals while at the same time the central computer accurately tabulates whichever directly connected terminal is being interrogated so that it is not necessary for the directly connected terminal to transmit an identification character associated with any messages originating at that terminal.

It is a still further object of the present invention to provide an improved integrated data processing system in which a plurality of remote terminals have access and can change information at a central computer only on a right-to-know" basis.

In accordance with one embodiment of the invention the data processing system includes a single transaction computer which performs the actual processing of the transactions and a control computer which transports information to and from the processing computer. Remote terminals are provided and are located within the plants according to various criteria. Different kinds of terminals are available in order that each particular department in which they are placed may perform its job more etliciently and economically.

Each terminal is connected to the computer or to one of the buffers by an information line and by a bit sample line. When the terminal requests service to transmit a message to the computer, bit sample pulses are supplied from the computer to the terminal to readout the bits of a character from the terminal onto the information line. When the character is received at the central computer it is immediately checked for validity and a response character is transmitted back to the remote terminal indicating that the transmitted character was valid or invalid. Each character transmitted in the system is followed by a response character, traveling in the opposite direction to indicate valid reception and to initiate transmission of the next message character.

Directly connected terminals are scanned in sequence and may only handle one message character per scan, per terminal. A two-speed scan is provided which interrogates each terminal for short intervals and spends more time with the terminal when it must be serviced than when it has no tratlic. The buffers as a group are interrogated between Bash sampling of 21 directly connected terminal and, if any require service then all buffers are interrogated at this time. While entering complicated messages into the system, the operator is assisted by the machine system in a process in which the operator and iachine system have an information interchange while a message is being entered. much as would occur between two humans performing the same task. This is referred to as a conversational mode of communication. Access to all information from the central processing unit is not provided for all terminals, since a transaction code plus terminal identification is utilized to insure access only a right-to-know basis.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 is a block diagram of the over-all system;

FIGURES 2A and 2B are logical diagrams of the remote terminal controls;

FIGURE 3 is a block diagram of the buffer used in the system;

FIGURE 4 shows the relation of the read and write heads to the various tracks on the magnetic drum butler:

FIGURE 5 shows diagrammatically the precession scheme used in transferring information to and from the buffer;

FIGURE 6 is a timing diagram depicting the operation of the butter;

FIGURE 7 is a diagrammatic showing of the order in which the buffered and directly connected terminals are interrogated;

llGURE ti shows the character registers and associated circuitry which receive characters which are to be transmitted between the central computer and the remote terminals;

FIGURE 9 shows the terminal sample controls at the central computer;

FlGURES 10A and 103 show the terminal group controls;

FIGURE 1 1 shows the six group controls;

FlGURE 12 shows the buffer sample controls; and

FIGURE 13 shows the right-to-know circuitry.

In the system transmission of messages is by 8-bit characters. The encoders and decoders shown in block form in the drawings are well known and many 8-bit encoders and decoders may be used for this purpose. The triggers shown in block form in the drawings are well known his*able devices which are turned off by an input to the left-hand input of the trigger and are turned on by an input to the right-hand input of the trigger. An input to the center of a trigger block signifies a binary input which switches the trigger to alternate binary conditions. Two outputs are provided on each trigger and the right hand output of the trigger is referred to as being up when the trigger is on and the left-hand input is referred to as being up when the trigger is off. When the opposite conditions prevail, the respective lines are referred to as being down or approximately ground. Conventional AND and OR circuits are shown in block form in the drawings. When all inputs to an AND gate are up the output is referred to as being up. When any one input to an OR gate is up the output is up. Conventional exclusive OR gates are shown in the drawings and are such that when either, but not both, inputs are up, the output is up. In referring to the drawings, the first one or two digits of each reference numeral denote the figure numbers on which the reference numeral appears.

Over-all system, FIGURE 1 Referring to FIGURE 1, there is shown a block diagram of the over-all integrated data processing system. In this system a processing computer 1 is used to perform the actual processing of the transactions with all required information brought to and taken from the processing computer as required. A control computer 2 is provided to control the flow of information from the remote terminals to the processing computer.

Several remote terminals, each of which provides access to the central computer and provides information from the central computer, are diagrammatically shown in FIG- URE 1. Some of these remote terminals are directly connected to the control computer. In the embodiment shown, these directly connected terminals are divided into six groups. A first group of directly connected terminals are designated A01, A02, A03 A20. A second group of directly connected terminals are designated B01, B02, B03 B20. The three groups C, D and E of directly connected terminals are not shown while the last group F01 F20 is shown. It will be understood that as many groups as are necessary may be provided and each group may have a varying number of directly connected termi nals.

The directly connected terminals are generally provided in locations which are in close proximity to the control computer. In outlying locations where extensive communication facilities are required to connect the terminals to the central computers, provision is made to connect the terminals to a buffer and thence to the control computer in order to conserve communication facilities. Terminals 3, 4 and 5 are shown connected through butter 6 to the control computer 2. Similarly, terminals 7, 8 and 9 are shown connected through the butter 10 to the control computer 2. Numerous other terminals may be provided at outlying stations and are connected through a plurality of buffers as required. It will be understood that the number of terminals serviced by one bufi'er will vary as required, and the quantity of butters can likewise be varied. A buffer is provided at each remote location and all of the terminals at that remote location are connected through the buffer to the control computer.

Each of the directly connected terminals is connected to the control computer by an information line and a bit sample line; the information line 11 and bit sample line 12 being shown connecting terminal A01 to control computer 2. Each buffer is connected to the control com puter by an input line and an output line; the input line 13 and the output line 14 being shown connecting buffer to control computer 2. Each bulfered terminal is con nected to the buffer by an information line and a bit sample line; the information line 15 and the bit sample line 16 being shown connecting terminal 7 to buffer 10. Information line 15 and bit sample line 16 are similar to their counterparts 11 and 12 from the directly connected terminals.

Terminal controls, FIGURE 2 The logic circuitry for controlling the flow of information to and from the remote terminals is shown in FIG- URES 2A and 2B. Each of the remote terminals, such as the terminals A01, A02, A03 or 3, 4 or 5 are provided with the controls shown in FIGURE 2. While the terminals controls will be described as being directly connected to the computer, the same controls are used at terminals connected through a buffer to the computer.

Each remote terminal is provided with a keyboard, or other input device 200 for entering information which is to be transmitted to the computer and an output mechanism 201 for registering information received from the computer. When the terminal is in a condition to transmit messages to the computer, the condition will be referred to as input and when the terminal is in a condition to receive messages from the computer the condition will be referred to as output.

Assume the use of a keyboard with this particular terminal. When the operator at the remote terminal wishes to transmit a message to the computer, he depresses the service request key on keyboard 200 which raises the SERVICE REQUEST line 202. All other keys will have been locked in their raised position. The SERVICE REQUEST line 202 acts through OR gate 203 to set the information line trigger 204 in the OFF condition. When the information line trigger 204 is set OFF, the line 205 goes down and, consequently, the output of AND gate 206 goes down. The output of AND gate 206 is connected to information line 207. When information line 207, which is connected directly to the central computer, is brought down to approximately ground level, the computer is signalled that one of the directly connected remote terminals requires service. As will subsequently be shown, in conjunction with the description of the central computer controls, the down condition of the information line together with the settings of certain control triggers, indicate a service request by the remote terminal.

The TRANSACTION CODE END trigger 253 is turned on by the SERVICE REQUEST 202 and turned ofi by the VALID RESPONSE to the TRANSACTION CODE. This is the first character following the SERV- ICE REQUEST through AND circuit 218 and OR circuit 217, which is gated by INPUT VALID RESPONSE line 216 from the RESPONSE DECODER 215. While trigger 253 is ON the keyboard is locked and remains locked for a short time after the TRANSACTION CODE END trigger 253 has been turned off. This allows the present signal on the INPUT VALID RESPONSE line 216 to fall.

Following a SERVICE REQUEST the keyboard 200 cannot be activated until the SERVICE ANSWER has been transmitted from the control computer or the buffer to the terminal to signify that the system is capable of handling this input message. The SERVICE ANSWER is an END OF MESSAGE character and is decoded at the output DECODER 243 to condition OR circuit 217,

AND circuit 218, KEY RELEASE LINE 219, AND circuit 220, and OR circuit 221. The output of OR circuit 221 conditions the SERVICE REQUEST KEY to raise it to its normal position. The keyboard will now also be unlocked to allow the entry of the message.

After depressing the SERVICE REQUEST key, the operator depresses the key of the first character in the input message. Depression of this key acts through encoder 214 to set the code of the desired character into shift register 208. Shift register 208 may, for example, be composed of magnetic cores. Each key from the keyboard 200 would then be wired through the four cores representing the four bits of this particular character.

Depression of the appropriate character key causes the bits to be set up in the shift register and locks the keyboard. The key returning to its normal position will raise the line 209 which acts through OR gate 210 to turn the INFORMATION LINE trigger 204 on. Line 205 from INFORMATION LINE trigger 204 goes up and this acts through AND gate 206 to raise the information line. The raising of the information line indicates to the control computer that a character has been entered into the shift register and this character should be read out. Accordingly, the CENTRAL CONTROL computer transmits a series of eight bit sample pulses which are received at the terminal over line 211. The first of these bit sample pulses on line 211 acts through OR gate 203 to turn the INFORMATION LINE trigger 204 off and to bring the information line 207 down. The incoming bit sample pulses are delayed for .25 millisecond by the delay device 212. These pulses are connected through an OR gate 213 to the SHIFT register 208 and shift the bits of the stored character out of the SHIFT register 208 onto the information line 207. The character is then transmitted to the computer.

When the computer receives the input character, the character is immediately checked for validity. If the transmitted character was a valid one, the computer sends a character back to the remote terminal indicating the reception of a valid character and enabling the remote terminal to proceed to send the next character. The character from the computer indicating that a valid input has been received is set serially into SHIFT register 208 and then decoded by decoder 215. When decoder 215 detects the character indicating a valid input, the line INPUT VALID response 216 is raised and this line acts through OR gate 217 and AND gate 218 to produce the KEY RELEASE signal on line 219. The KEY RE- LEASE signal acts through AND gate 220 and OR gate 221 to unlock the keyboard and permit the entry of another input character. The keyboard is unlocked and upon depression of the next key the steps described above are repeated.

During the transmission of an input character, the shift pulses fed through OR gate 213 to SHIFT register 208 are monitored to insure that the shifting is accomplished properly. Shift pulses from OR gate 213 are also fed to a PARALLEL SHIFT register 222. When PARALLEL SHIFT register 222 has received the proper number of shift pulses to read a character out of SHIFT register 208, the SHAFT register 222 is set to the home position, thus raising the line 223. This condition should occur within a predetermined time interval and if it has not occurred in this time interval there has been a shift error. In order to detect this time interval, a SINGLE-SHOT multivibrator 224 is provided. This single-shot is set upon the occurrence of the first shift pulse from DELAY DEVICE 212 and is reset a predetermined time period thereafter. The output of single-shot 224 is gated against the HOME POSITION signal on line 223 in the EXCLU- SIVE OR gate 225. An output of EXCLUSIVE OR gate 225 is indicative of a shifting error. The output of EXCLUSIVE OR gate 225 actuatcs PULSE generator 226 which produces auxiliary bit sample pulses which act through OR gate 213 to clear the SHIFT register 208 and 7 to drive the PARALLEL SHIFT register 222 to the home position. The output of EXCLUSIVE OR gate 225 also acts over line 227 to turn on the SHIFT ERROR trigger 228. The output of EXCLUSIVE OR gate 225 also acts through OR gate 210 to turn the INFORMATION LINE trigger 204 on, thus bringing the information line 207 up.

The SHIFT ERROR trigger 228 is set ON by the output of EXCLUSIVE OR gate 225 and is set OFF by the END OF BIT pulse from the single-shot 224. When SHIFT ERROR trigger 228 is set ON, the output of AND gate 229 goes down and this signal, when inverted in inverter 230, inhibits the encoder 214. The output of inverter 230, denoted INPUT SHIFT ERROR, is also applied through OR gate 232 to set trigger 233 into the ON condition. When trigger 233 is in the ON condition, the line 234 is down. Line 234, denoted RE-ENTER INPUT CHARACTER, is up when trigger 233 is set ino the OFF condition by the END OF BIT signal from the single-shot device 224.

When the remote terminal transmits an invalid character to the central computer, the circuitry at the com puter recognizes the character as invalid and the response character transmitted to the remote terminal over information line 207 is indicative of the invalidity of the input character. The response character is set into shift register 208 and decoded in response decoder 215 which in turn raises line 236, INPUT INVALID RE- SPONSE. This signal acts through AND gate 237 if the service request line 202 is up, and OR gate 221 to release the keyboard to permit the character to be re-entered. 5

The keyboard remains locked until a VALID RE- SPONSE is received. To prevent the keyboard from remaining locked indefinitely, in the case where a wrong character has been set up in the register, AND gate 237 is used to feed a signal to OR gate 221 if the SERV- ICE REQUEST LINE 202 goes up at as result of the operator depressing the SERVICE REQUEST KEY on the keyboard 200 while INPUT INVALID RESPONSE 236 is up.

When the operator has inserted the complete input message, he depresses a key which indicates an INPUT END OF MESSAGE. This key raises the line 240, input EOM, which acts through OR gate 241 to change the condition of the OUTPUT CONDITION CONTROL trigger 242. The outputs of OUTPUT CONDITION CONTROL trigger 242 are connected to OUTPUT decoder 243 and to RESPONSE decoder 215 so that RE- SPONSE decoder 215 is now de-conditioned and OUT PUT decoder 243 is conditioned to decode an output message. The terminal is now in condition to receive characters from the central computer. These characters are received on information line 207 and set into shift register 208 by the shift pulses. The characters set into shift register 208 are decoded by OUTPUT decoder 243 which in turn actuates the appropriate character of the OUTPUT mechanism 201 which may, for example, be a print hammer or a punch for that particular character. Each setup of the OUTPUT mechanism 201 is sensed to detect whether the output setup is valid or invalid. If the output setup is valid, the line 244 is raised and the output of AND gate 245 is raised to encode a valid setup character in the encoder 214. This valid setup character is set into shift register 208 and transmitted on information line 207 to indicate to the computer that the output character has been received and that another output character should be transmitted. If the OUTPUT mechanism 201 detects an invalid setup, the line 246, INVALID SETUP, is raised ad this acts through OR gate 247 to enable encoder 214 to encode a character indicating that the output character should be retransmitted. The encoded character from encoder 214 is set into shift register 208 and transmitted over information line 207 to the computer to indicate that the last output character is to be retransmitted.

Means are provided to detect a shifting error on output 8 similar to the means for detecting an input shift error and condition AND gate 245. OUTPUT MODE CON- TROL trigger 249 is conditioned in accordance with whether the terminal is in the input or output mode. If a shift error, indicated by the setting of SHIFT ERROR trigger 228 occurs at the same time that the OUTPUT MODE CONTROL trigger 249 is set, AND gate 250 is energized and the output of this AND gate, OUTPUT SHIFT ERROR, acts through OR gate 247 to enable encoder 214 to encode a character indicating that the last output character has not been received properly and that it should be retransmitted.

OUTPUT mechanism 201 requires a slightly longer perod of time to set up certain output characters. For example, FORM STOP, TAB AND LINE SPACE re quire slightly extended periods and the remote terminaI can not accept another output character immediately after the reception of one of these characters. When one of these characters is being set up, the line 251 is raised, the output of inverter 252 goes down, and the output of AND gate 206 goes down, thus bringing information line 207 down. When information line 207 goes down, the central computer is signaled to delay the next output character until the action is complete and the information line 207 has gone back up.

When the message from the computer to the terminal is completed, the computer transmits an output end of message character. This output end of message character is decoded by the OUTPUT decoder 243 and the line 252 is raised. This line, acting through OR gate 241, changes the condition of the OUTPUT CONDITION CONTROL trigger 242. The output EOM signal also acts through OR gate 217, AND gate 218, AND gate 220 and OR gate 221 to release the keyboard 200 and permit another input message to be entered. The output of AND gate 218 sets the TRANSACTION CODE END trigger 253 in the OFF condition and OFF output of this trigger acts through delay 254 to enable AND gate 220, thus providing a key release for the keyboard as previously mentioned, except on an end of tranaction. When an END OF TRANSACTION key on keyboard 200 is depressed by the operator, the entire keyboard 200 except the SERVICE REQUEST key, is locked against an operator entering any additional information. The terminal controls are then left in an output condition. Line 207 fed by AND gate 206 is brought up. The IN- FORMATION LINE up with the TERMINAL in an OUTPUT CONDITION and the IN USE trigger o-lf signifies that the terminal is awaiting a new transaction.

Terminal buffer, FIGURE 3 The controls described above were described in conjunction with the terminals which are directly connected to the center computer. The controls described above can also be used for the terminals which are connected to the computer through a buffer. The buffer and associated circuitry, shown in FIGURE 3, will now be described.

In the embodiment shown, the buffer is a magnetic drum 301. The magnetic drum 301 is divided into five sections. The section 302 is provided to process input messages transmitted from the terminals to the computer. The signal section 303 is provided to actuate the switching for positioning of successive characters and indicates the input and output modes of the terminals. An output section 304 is provided to process output messages transmitted from the computer to the remote terminals. The sector timing section 305 indicates the relative location of the drum in its total cycle and makes possible resynchronization if the buffer or terminals fall out of step with the rest of the system. The bit timing section 306 is provided to generate bit sample pulses to read information into and out of the input shift register at each terminal.

On the terminal side of the buffer there is shown diagrammatically a scanning switch 307 for selectively scanning all of the remote terminals connected to the buffer. The scanning switch selectively and successively connects a plurality of pairs of transmission lines to the buffer. In the embodiment shown, five pairs of lines, corresponding to five terminals being serviced by that butter, are provided. Of these pairs, the lines 308, 309, 310, 311 and 312 are information lines and are connected, for example, to the information line 207 shown in FIGURE 2. The other lines 313, 314, 315, 316 and 317 are bit sample lines and are connected, for example, to the bit sample line 211 in FIGURE 2. As the scanning switch 307 moves from position-to-position, one of the information lines, for example, 312, is connected to either the input section 302 or the output section 304 of the buffer depending upon whether the terminal is in the input or output mode. Also, one of the bit sample lines, for example, 317, is connected to the bit timing section 306 of the drum in order to provide bit sample pulses to the remote terminal.

Each of the remote terminals connected to the butter is provided with twenty-character sectors in the input 302 and output 304 sections of the drum. A twentyfirst, or dummy sector, is also provided to allow the processing scheme to be described later. The remote terminals are connected to the buffer through a scan control and decoder 318 which is operated under control of the sector timing section of the drum 305. This insures that the terminal is connected to the appropriate sector of the input or output sections of the drum.

On input, the remote terminal is connected to one of the two CHARACTER registers 319 and 320. Successive characters are read into alternate ones of the CHARACTER registers 319 and 320. The provision of these two CHARACTER registers allows the reading out of one CHARACTER register while the next character is being inserted into the other CHARACTER register.

On input, the characters of a message are stored in one of two subsectors of the sector assigned to that terminal. The characters are gated from the CHARACTER registers 319 or 320 to the appropriate subsector by the decoder 321 which is operated under control of signals from the signal section 303 of the drum. The message characters are transmitted to the computer by reading the characters from the input section 302 under control of decoder 322 which again is controlled by signals from the signal section 303 to insure that the proper subsector of the drum is read out. The characters are transferred to one of two CHARACTER registers 323 or 324 and thence over line 325 to the computer. Transmission between the butter and the computer is independent of the exact time that transmission is taking place between the buffer and the terminal.

On output, message characters are transferred over line 326 from the computer to one of two CHARACTER registers 327 or 328. As before, two CHARACTER registers are provided so that one register can be read while the next character is being set into the other register. The characters are transferred through decoder 329 to the sector of the output section 304 which is assigned to the terminal to which the message is addressed. At the appropriate time the message is read out of output section 304 to one of two CHARACTER registers 330 or 331 and then transferred through scan control and decoder 318, through the scanning switch 307 and over one of the information lines 308312 to the addressed terminal.

The organization and operational details of the buffer may be better understood with reference to FIGURE 4 which shows a section of one terminal buffer drum track. This diagram gives an indication of the relative position of the heads with respect to the sectors on each of the five tracks. Assume the heads to be stationary and the scale under them representing the various tracks as moving to the left. There is shown in FIGURE 4 a read head 401 for the bit timing track 306, a read head 402 for the sector timing track 305, a write head 403 for the output track 304, a write head 404 for the signal track 303, a read head 405 for the signal track 303 and a read head 406 for the input track 302. The heads on the termiral side of the bufier are shown in FIGURE 4. It will be appreciated that identical heads will be provided for three of the five tracks on the computer side of the drum.

The timing bit rate track 407 provides pulses for each of the bit positions in each sector. There is an interval of 0.10710 degree between the center of each of these bits on the surface of the drum. These, as well as the timing sector rate, are permanently recorded.

The timing sector track 408 is required for proper positioning identification in case the drum is ever out of step with either a terminal or the control computer. All the information that is required here is provided by signals of three different durations. A constant signal is provided for approximately the last quarter of each sector with the execution of the dummy sector No. 21. The fall of the signal indicates the beginning of a new sector. During the last quarter of the first subsector of each sector there is another constant signal to indicate the start of the second subsector. The entire first subsector of the dummy sector has a constant signal to provide the indication of the beginning of the series of sectors. Two single-shot circuits are used to recognize the various durations of these three signals. Characters may be transmitted only if the buffer is in proper synchronization.

An output track 409 and an input track 410 are provided for output and input messages, respectively. These tracks will normally be reset to a character configuration representing a blank character. Read and write heads are associated with the proper side of the drum. The heads are a specific distance from each other and have a constant relation with the heads on the timing tracks.

The remaining track, the signal track 411, requires four heads positioned so that the tracks read head is able to read the entire sector before the heads for the input and output tracks have reached the same sector. In order that there be time to set the switches and because every fifth sector is serviced each revolution, the read head on the terminal side precedes the write head by six sectors. The head position on the computer side is not critical as eleven revolutions are devoted to each sector serviced. The write head must be positioned so that it will not start writing until the sector on the information track has been completely passed. Again, spacing between the heads on this track must be accurate with respect to each other and there must be a constant relation with respect to the heads on the other tracks. The portion of the signal track associted with each terminal will first indicate the fullness condition of each subsector. The portion is indicated as empty until it has been filled and as filled until it has been completely emptied. The characters are stored in the proper subsector as well as in the proper position within the subsector. It is not sufiicient to choose the first open position as this might be the first position of the first subsector when the second subsector is the one currently being filled. This occurs when characters have just been sent to the control computer from the first subsector and it is indicated as empty while the second subsector is being filled. In order to insure both the proper subsector as well as the proper position within the subsector, a character count is stored on the signal track. Positioning information of this type is required for both the terminal and the computer sides of the buffer. This information must be available before the terminals input and output sectors have reached their respective heads.

The character counts on the signal track are immediately followed by the terminals identification character. This is a single character that is transmitted to or from the control computer preceding each group of ten characters to be transmitted. The signal track requires a read head on the terminal side of the drum that is ahead of the reed head of the output track and write head of the input track. A write head is positioned so that the entire sector will have passed the heads on both the input and output tracks before the same terminals sector reaches the write head on the signal track. The same configuration is duplicated on the computer side of the buffer.

All transmission between the buffer and the central computer will be in groups of ten characters each. Thus, it is possible to detect at either end of the transmission line if any characters have been added or deleted in error during transmission. To implement this scheme, means are provided to transmit blank characters whenever the end of the message will not complete a tencharacter group. To accomplish this, each character position on the information tracks is reset to a blank character after the character that had occupied the position has been successfully transmitted.

Each revolution of the drum, which in the embodiment shown, occupies 10.5 milliseconds, allows a single character to be transferred to or from the central computer. However, each of the twenty terminals must be serviced at a rate of approximately twenty characters per second. Thus, a precessing scheme is used. With the sectors located sequentially around the drum, the terminals 1, 6, 11 and 16 and the dummy position are sampled during the first revolution of the drum. The second revolution services terminals 5, 10, and 20. This precessing scheme may be followed on FIGURE 5. It should be noted that the next sector to be serviced may be found by adding 5 modulo 21 to the sector presently being serviced.

Operation of buffer The exact timing relationships for the operation of the buffer can best be seen in FIGURE 6. The terminal operations previously described in conjunction with the unbufl'ered terminal, FIGURE 2, will apply here.

When the operator has requested service, the service request key on the terminal remains latched down and the information lines 308, 309, 310, 311 or 312 to the butter will fall. If the terminals sector on the signal track indicates that the terminal is not in use, the buffers circuitry will cause a service request and an end of message character to be written on the input track 302. The bit positions for writing these characters and all other operations are controlled by the bit timing track 306. Writing the end of message character causes the indicattion on the signal track to show that this subsector is full and that the terminal is in use with an input message. The remaining eight positions in the subsector contain blanks.

The computer side of the buffer will now act as it does for any input message. As soon as this sectors turn to be serviced occurs, the computer interrogates the signal track. As soon as the bufier has completed the transmission for an entire subsector, the butter interrogates each of the successive sectors to find the next sector requiring service as designated by the full subsector indication. The buffer will first transmit the terminals identification character, followed by all ten characters on this subsector, even though in this case the only message character is a service request. The buffer is now prepared to service another terminals sector while the control computer is determining if it can service the first terminal.

When the control computer is ready to send the service answer, it signals the buffer to pick up the message composed of the terminal identification, service answer, end of message, and eight blank characters.

This message is placed on the output track -4 in the sector designated by the identification code and an indication will be made that the subsector is full.

The service answer may now be sent from the bulTcr to the terminal. As four terminals are serviced each till revolution, the initiation of this transmission, at the most, will take five revolutions. As in the unbufi'ered case, the validity will be checked on a character-by-character basis with the terminal returning the response for the previous character just preceding the reception of the new character. As this is the first character in the group of ten, the response preceding it will automatically indicate valid.

The signal track is used to indicate from which position in the subsector the character will be removed or added. This indication will always be for the last character transmitted and a valid response will cause the next character to be removed from or added to one position higher. Thus, if the character becomes invalid during output transmission, the original character will be retransmitted.

The end of a message character is transmitted in a similar manner. The buffer will, however, not transmit the blank characters following the end of message to the terminal. The signal track will be automatically changed to indicate the terminal as now being in an input mode with the subsector empty and ready to receive the next message.

The transaction code and an end of message character will now be entered at the terminal. As the terminal is now indicated as being in use, the information line will be brought up to indicate that a character is ready to be transmitted. The information line must go down as soon as it has received a valid response. The signal track will indicate that the first character shall be written in the first position of the first subsector. The characters will be transmitted serially from the terminal and entered serially onto the input track 302.

The validity of each character will be checked immediately by the buffer and the response, returned to the terminal. If the response is valid, the character position counter on the signal track will be increased by one and the terminal will allow the next character to be entered. The end of message character will however, keep the terminal locked out against any additional characters being entered at this time.

The end of message character at the buffer will cause the subsector to be indicated as full so that the comouter side of the buffer may now transmit the transaction code to the control computer in the same manner as it previously transmitted the service request. When the control computer has determined that it may accept the transaction it will return the service answer as previously described following the service request.

On input to the computer, the service answer will be returned to the terminal to unlock the keyboard and to allow the input message to be entered. The procedure here will be the same as for entering the transaction code except that now, presumably, the message will be longer than a single character group. As soon as all ten character positions have been correctly filled, the signal track will indicate that the first subsector is full and immediately allow the next character to be entered in the second subsector. The ten characters will be transmitted to the control computer as soon as possible so that the first subsector will be available for the succeeding group. The only time when there can be a delay is if the butter is overloaded and cannot transmit to the control computer as rapidly as the terminals are providing it with information.

If the terminal is not prepared with a character when it is sampled, no information can be removed, but the full 2.5 milliseconds assigned to this terminal will not be able to be used for anything else. Characters will be entered and transmitted until the end of message character is received. Then, as before, the keyboard will remain locked until the control computer sends instructions to unlock it.

Output messages from the computer to a remote terminal can be initiated by a processing program or follow an input message from the same terminal. In either case, the central computer must find that the butter is free and that the terminal is in an output condition. The terminal could only be in an input condition if this output message were initiated by the processing program just as a service request was being entered at the terminal.

For each ten character groups the terminal identification will be transmitted first in order to set up switching at the buffer to allow the characters to be stored on the output track in the proper sector. The validity of each character will be checked as received and if any character is invalid it and the remaining ones from the group of ten will not be stored and the invalid response will be returned to the central computer to request retransmission of the entire group.

As soon as all characters have been validly received, the signal track 303 indicates the subsector as being full and the next time the terminal side of the butter services this sector the first character will be removed. These characters will be transmitted to the terminal as previously decsribed for the service answer.

An end of message character will act as previously described and ultimately allow the terminal to enter additional characters. An end of transaction, however, will be transmitted in the same manner but will keep the keyboard locked. The end of transaction will always be initiated by the processing program.

If the terminals actually transmitted a character every 2.5 milliseconds, the computer side of the buffer would only be able to handle four terminals. The input rate to the buffers will never be this high. The buffer will always choose the next sector requiring service and will continue with it until either subsector is full and ready to transmit. The central computer and buffer will both be required to send the terminal identification preceding each group of ten characters and receive the validity response following the last of the ten. A character count will always be made in computer transmission with the butter to insure that exactly ten characters (including any blank characters required) and thus only the intended messages, are transmitted in each group.

If the traffic through the buffer develops a heavy peak, the use of two subsector-s for each sector will act as a smoothing device for the terminal. The subsectors must be emptied in the proper sequence as indicated by the character counter on the signal track. If this indication were not available, reading and writing would always start with the first available subsector of the two subsectors. However, this could cause portions of the message to be out of sequence if, and only if, the following two conditions exist: first, if the second subsector is only partially filled while the first subsector is being emptied and secondly, if the buffer becomes excessively busy so that the filling of the second subsector is completed and then the first subsector again is filled before the computer side of the buffer is able to read from this terminals sector.

While the buffer as described cannot handle the maximum possible traflic for all its terminals, it is considered that it will be much more than adequate for its anticipated loads.

Terminal interrogation equipment, FIGURE 7 The terminal interrogation equipment is that portion of the terminal control system located in the control computer 2 that determines which terminals require service and identifies to which or from which the messages must flow.

Each of the 120 unbutfered terminals must be interrogated in sequence, and, if the interrogation shows that the terminal requires service, it must be serviced. A second bit sequence for servicing the four buffers must be interspersed with this first sequence. The system will allow 2.5 milliseconds for the transmission of each charac ter with a terminal and each terminal must be interrogated approximately every 50 milliseconds. These conditions require a two speed scan to be used. That is, a busy terminal will be allowed its full 2.5 milliseconds while a terminal that is not in use should require only a single machine cycle which is assumed to be 10 microseconds. This interspered, two-speed scan may be more readily understood by reference to the diagram of FIGURE 7. Starting at the top of this diagram, all four of the buffers are scanned to determine if any of the buffers require service. If any of the butters require service, then all four bufiers are interrogated after which the next terminal is scanned to determine if it requires service. If none of the buffers requires service the next terminal is scanned immediately. If the next terminal does require service, a 2.5 millisecond terminal interrogation cycle is provided after which the scanner steps to the next terminal at the end of 10 microseconds. If the next terminal does not require service the scanner steps to the next terminal at the end of 10 microseconds. Before scanning the next terminal, all four of the buffers are again scanned to determine if any of the buffers require service. The logic circuitry required to implement this scanning will be decsribed subsequently. The operation of the control computer in scanning the unbuffered terminals and the operation of the computer in scanning the bufiered terminals will be described separately before describing the interspersed scanning of buttered and unbuflered terminals.

Control computer, working memory, character register and associated circuitry, FIGURE 8 The directly connected terminals have been divided into six groups of twenty terminals each as shown in FIGURE 1. Each group of directly connected terminals has two single character registers associated therewith. As shown in FIGURE 8, CHARACTER registers 801 and 802 are associated with Group A, CHARACTER registers 803 and 804 are associated with Group B and CHARACTER registers 811 and 812 are associated with group F, the CHARACTER registers associated with groups C, D and E not being shown in FIGURE 8. These single CHAR- ACTER registers provide temporary storage for messages being transferred between the working memory 813 of the control computer and the information lines to the directly connected terminals.

Each group of directly connected terminals is sampled sequentially and within each group each terminal is also sequentially sampled. Thus, each terminal is selected in order. The sampling sequence is: A01, B01, C01, D01, E01, F01, A02, B02, C02 D20. E20, F20, A01, B01. Because the terminals are always interrogated in accordance with this pattern, it is not necessary to transmit any terminal identification as the computer always knows which terminal is being interrogated.

Associated with each pair of character registers is a decoder. For example, decoder 815 is associated with CHARACTER registers 801 and 802 and provides indications when an output EOM, or an input EOM is received. The decoder also provides an indication, CHARACTER REGISTER OK, when a CHARAC- TER register is available for use by one of the terminals. Also associated with each group of terminals is a TERMINAL AVAILABLE trigger which indicates whether another terminal may be serviced when it requests service. For example, the TERMINAL AVAILABLE TRIG- GER 816 is provided for the A group of terminals. Also shown in FIGURE 8 is the computer 814 which provides numerous signals to the terminal system. As an example, the lines 817, 818 and 819 are shown. These lines are up when the computer program has initiated an output message for terminals A01, A02 or F20 respectively. The remaining output control program lines are not shown.

The direction center for all operations involving the terminal system will be a Terminal Table. It consists of two major portions; the triggers and information lines which will be subsequently described and a magnetic core memory which is part of working memory 813. The portion of the terminal table in working memory can best be thought of as a matrix with each row representing one of the terminals. The columns of the matrix represent the transaction code, usage number, address, availability, priority, location, and time.

A transaction code is the unique identification assigned to each of the individual processing programs. They are used to obtain the proper processing program from program storage.

As the transaction code from a terminal is only a single character, it cannot alone provide sufficient identification to find the proper processing program. The control program will have to take both this single character transaction code and the terminal identification number to provide complete identification for the table look-up procedure. This will have the added advantage of aiding any so called rightto-know procedures as the use of any particular transaction program will be restricted to a very definite group of terminals. That is, a clerk at the shipping dock will have absolutely no access to the programs and information concerning the cashiers office. This can also be used by the control programs as an aid in detecting an obviously incorrect transaction code.

A control program will be required to reset the transaction code column for a terminal to a no character character. This will be used by another control rogram to signify that a terminal has been granted service but as yet has not entered the transaction code.

The usage numbers provide identification for each of the processing programs in the computer at any one time. For example, if there are several SALES ORDER EN- TRY PROGRAMS being processed at one time the usage numbers would be the only possible means of differentiating from one another. Usage numbers are of the greatest importance while checking on on the availability of a terminal for an output message. They can be any closed sequence of numbers and have here been assumed to be from one to one thousand.

The ADDRESS COLUMN of the TERMINAL TABLE will contain the address in working memory of the location where the next input character is to be placed or the location from where the previous ouput character was obtained. The difierence at the control computer occurs as a result of the diiference in time in receiving response characters. The input characters validity is checked immediately upon the receipt of the character and thus the memory location can also be immediately updated. On the other hand, the response character for output messages is not ready at the terminal until the next character is about to be sent from the control computer. If a valid output response is received at the control computer at this time. the address in the TER- MINAL TABLE will be changed to indicate the next character. In either case, if the character transmitted was received in an invalid form, the address will remain the same and either the repeated input character will be stored in place of the invalid one or the transmission of the output character will be repeated.

Between transactions, the ADDRESS COLUMN in the TERMINAL TABLE is reset to the constant location of the fixed character that represents the service answer (EOM). It should be recalled that the first charatcer to be transmitted following a service request must always be this service answer. It is then necessary for the address to be changed to that of the TRANSACTION CODE COLUMN for this particular terminal so that the transaction code may be stored. This will be accomplished by one of the control programs.

The PRIORITY COLUMN servse several functions in the control computer. First, its contents will be used to indicate a relative priority for determining the latest time by which an OUTPUT message must be transmitted. This is accomplished in either of two ways. If an output message for a transaction originated by the operator is to be transmitted, the priority will be the sum of a constant stored with the transaction program and the time at which the input message was entered. If an output message for a transaction originated by the control computer is to be transmitted, the priority will be a constant stored with the transaction program representing the time each day by which this transaction must be accomplished. An example of this would be any transaction that must be accomplished ANY time between the end of one work day and the start of the next.

The second function of the quantity in the priority column is, to provide a safeguard so that no INPUT message, once initiated, is left uncompleted. Again the priority will be the sum of a constant stored with the transaction program and, in this case, the time at which the transaction code is entered into the control computer. This entry of the priority in the TERMINAL TABLE will be accomplished by the same control program that identifies the transaction code.

Another control program will periodically inspect the PRIORITY COLUMN to determine if any of the messages have been delayed excessively. Output messages will be transmitted as soon thereafter as possible and a message will be sent to the supervisory station so that human action may be initiated to discover the cause of the delay in the completion of the entering of the input message.

The LOCATION columns contain the beginning address of a PROGRAM LOCATION TABLE and the end address expected for an input message. This is used to insure that an input message is not longer than the room set aside for it. If it were longer, it could write over and destroy any record that has been written in working memory beyond it.

The AVAILABILITY column is used to signify that a required terminal is currently busy. This will normally be of consequence only when the computer Wishes to initiate an output message while a terminal is transmitting a difierent input message or involved with another transaction.

The TERMINAL IDENTIFICATION NUMBER COLUMN is used to provide terminal identification of messages stored in working memory and with messages sent to the buffers.

Control computer terminal sampling logic circuitry, FIGURE 9 Referring to FIGURE 9, there is shown the logic circuitry at the control computer associated with each terminal. The circuitry within the block 901 is associated with the terminal A01. Only the logical circuitiy associated with TA01 is shown in detail. It will be understood that the logical circuitry 902 associated with TA02, the logical circuitry 903 associated with TB20 and the logical circuitry 904 associated with TF20 are all identical. One hundred and twenty identical logical circuits are provided and are associated with each of the one hundred and twenty unbutfered terminals in the system. Referring to the details of the logical circuitry 901, an IN USE trigger 905 indicates whether terminal A01 is currently being used to transmit an input or an output message. An OUTPUT CONDITION trigger 906 indicates whether the terminal is currently in an input or output condition. When trigger 906 is on it designates an output condition and when it is off it indicates an input condition. OUT- PUT CONDITION trigger 906 is turned on by the input EOM from decoder 815 and is turned otf by the output EOM from the decoder 815. The normal or reset condition of OUTPUT CONDITION trigger 906 is on. This indicates that the terminal is in an output mode Whereas the off condition indicates an input mode. The IN USE trigger 905 will initialy be turned on either by a service request or, in the case of the computer initiated transaction. by the output control program. Both the SERVICE REQUEST signal and the OUTPUT CONTROL PRO- GRAM TA01, line 817, are connected to OR circuit 907 and then to the on input of trigger 905. The IN USE 

